1. Field of the Invention
The present invention relates to a sheet feeder used for an image forming apparatus such as a printer, a copier, a fax machine, or a multi-functional peripheral incorporating these functions. The present invention also relates to an image forming apparatus including the sheet feeder.
2. Description of the Related Art
To date, only sheets of high-quality paper, plain paper specified by copier manufacturers, or the like have been used as sheet recording medium that can be continuously fed in image forming apparatuses such as printers, copiers, and fax machines. Such sheets of high-quality paper, plain paper, or the like have low surface smoothness, whereby their inter-sheet adhesion is comparatively low. Thus, it has been comparatively easy to prevent double feeding that may occur when the cut sheets are fed out one at a time from a sheet loading section such as a sheet feed tray. The term “double feeding” refers to a phenomenon in which a plurality of cut sheets adhering to each other are simultaneously fed out. Moreover, even if double feeding occurs when such cut sheets are used, it is possible to separate doubly fed cut sheets by providing a separation roller, a separation pad, a separation claw, or the like to the sheet feeder so that the cut sheets can be smoothly fed one at a time.
However, the sheet recording medium has become diversified in recent years. Sheets having a low surface smoothness such as high-quality paper, plain paper, or the like are not the only sheets used as sheet recording medium. In particular, as the colorization technology for image forming apparatuses has improved, a paper having a high surface smoothness such as a coating paper can now be used. A coating paper is composite paper of which one or both sides are coated with a coating color, which is a coating material, so as to improve printability. A coating paper has a high whiteness and gloss. Thus, in recent years, demand has been increasing for feeding not only high-quality paper and plain paper, but also the above-described coating paper, film sheets, tracing paper, and the like in an image forming apparatus. Because coating paper, film sheet, tracing paper, and the like have a high adhesion between papers, it is difficult to prevent double feeding of such sheets. Therefore, it is necessary to introduce special measures in order to feed (in particular, to feed out) such sheets.
Moreover, a stack of sheets loaded on a sheet loading section is prone to absorb moisture because the upper surface and the outer periphery of the stack of sheets are exposed to the air outside. The upper surface and the side surfaces of the stack of sheets absorb moisture and swell, while the inside of the sheet stack swells to a lesser extent because the inside absorbs less moisture than the upper surface and the side surfaces. As a result, inner spaces of the sheet stack (spaces between sheets) enter a negative pressure state, which causes the sheets to adhere to each other.
In order to reduce adhesion between sheets and separate the sheets in a sheet stack before feeding the sheets, some large copiers and the like adopt sheet feeders including mechanisms (hereinafter referred to as “side warm-air assists”) for blowing warm air toward side surfaces of sheet stack.
For example, there is a known technique that increases the efficiency of sheet separation while fulfilling the requirement for reduction in size and power consumption. With this technique, movement speed of an air shielding member, which serves to partially close an opening through which blowing means blows air from an outlet thereof toward a side surface of a sheet stack, is changed so that air is effectively blown toward an upper part of the sheet stack.
However, with this sheet separation technique, for example, while a large number of sheets are being continuously fed, sheets in a lower part of a sheet stack may be fed without being separated and may cause jamming. This problem is particularly serious when art paper or coated paper, which has high inter-sheet adhesion, is used in a high-humidity environment.